In desirable circumstances, materials added to plastic or coating formulations to achieve one beneficial result can produce another detrimental result, such as diminished mechanical properties from thermal oxidation degradation of the polymer. Thus, desired benefit of the additive together with the desired mechanical strength cannot be achieved simultaneously. Such is the case with additives for laser marking or coloration, such as the incorporation of certain colored inorganic metal oxides, metal compounds, and mixed-metal oxide pigments, the additive materials necessary for laser direct structuring and for other value-adding additives. In these noted instances, it is desirable to have a method to incorporate these materials into plastic and coatings or paint formulations while protecting the resulting materials from oxidative damage.
Inorganic colored pigments are based upon crystalline materials comprised of oxides of mixed metals. The materials are generally described in the published brochure of the Dry Color Manufacturer's Association (DCMA), now called CPMA, of 1117 N. 19th St., Suite 100, Arlington, Va. 22209, (1991), entitled Classification and Chemical Descriptions of the Complex Inorganic Colored Pigments, see particularly pages 9 to 35.
Laser Direct Structuring (LDS) refers to the process of selective metallization of defined regions after impingement of laser irradiation “activation” on a substrate containing an additive that releases a metal “seed” capable of promoting the deposition of metal atoms from solution to form a conductive pathway. The practice is disclosed in U.S. Pat. No. 7,060,421, Naundorf, et al., issued Jun. 13, 2006. In an LDS process, a computer-controlled laser beam travels over the molded thermoplastic article to activate the plastic surface at locations where a conductive metal path is desired, thus, “structuring” the article. With a laser direct structuring process, it is possible to obtain small conductive path widths (such as of 150 micrometers or less). Conductive paths are realized after structuring of thermoplastic surface and subsequent exposure of the structured article to a metal plating bath or series of metal plating baths.
The term “plastics” covers a range of synthetic or semi-synthetic compounds that are the products of polymerization reactions. They are composed of organic condensation or addition polymers and may contain other substances to improve performance or durability or other advantageous benefits. There are a few natural polymers generally considered to be “plastics”, however, most are synthetic organic derivatives. Plastics are formed into sheets, objects, films, laminates, and fibers. They are very versatile in processing, offering a very broad applicability in diverse markets.
Plastics can be classified in many ways, but most commonly by their polymer backbone and material properties. Thermoplastics can be repeatedly melted and reshaped, whereas processing of thermoset plastics produces crosslinks that cannot be reversed. Both materials have a plurality of uses. Examples of thermoplastics are polyvinyl chloride, polyethylene, polymethyl methacrylate, and other acrylics, olefins, aromatics, silicones, polyurethanes etc. Other classifications include thermoplastic resins, engineering plastics, and products of addition or condensation reactions. The so-called thermoplastic rubbers and thermoplastic elastomers are also included in the definition of thermoplastic resin. Thermoplastic resins per se include polyolefins, polystyrenes, polycarbonates (PC), polyesters, acrylonitrile butadiene styrenes (ABS), ABS copolymers, polyvinyl chloride (PVC), unplasticized polyvinyl chloride (UPVC), polyphenylene oxide (PPO), polyamides, polyurethanes (TPU), acrylic polymers, polysulfone polymerss and others.
The term “thermoset or “thermoset plastic” or “resin”, as used herein, refers to any plastic that can be formed into a shape during manufacture, but which sets permanently rigid upon further heating. This is due to extensive cross-linking that occurs upon heating, which cannot be reversed by reheating. Examples include phenol-formaldehyde resins, epoxy resins, polyesters, polyurethane, silicones and combinations thereof. Thermoset resins most often used in the present invention include epoxy resins (“epoxies”), polyimide resins (“polyimides”), bismaleimide resins (e.g., bismaleimide trizaine (BT)), and combinations thereof. Additionally, the term “resin” may refer to any reactive monomeric, polymeric, and/or functionalized material that hardens to form a film or “coating” commonly utilized in compositions of paints, lacquers, varnishs, or emulsion utilized for forming a decorative or functional coating on an article. Noted examples of coatings resins, include alkyds, acrylates, epoxies, urethanes, polyesters, and many hybrid systems that are compounds or mixtures or the like, for example perfluoro-acrylates, or siliconized polyesters. Thermoset elastomers are a unique class of resins and are included in the definition of thermoset plastic as well as, analogous relationship for elastomeric resins employed in coatings.
The definition of plastic additive is any material that is added to a plastic or polymer to enhance or modify their original physical and/or chemical properties. The definition of filler, in the sense relating to plastics, is a plastic additive, solid substance, which is added to a polymer to displace plastic material. Inclusion of fillers can positively or negatively modify plastic properties. Often they are inert inorganic or cellulosic materials. Additives are used extensively to reduce cost, improve mechanical properties, impart color, enhance processing, or add new functionality, such as, laser marking and other beneficial attributes known to one skilled in the art. The effect of fillers on the mechanical properties of polymers depends on their specific surface area, particle size, shape, and reactivity with the plastic matrix.
According to the prior art for laser marking, a plastic or coating composition can be adjusted in such a way that it can be inscribed by laser light, whether by choosing a grade of plastic having good laser inscription properties or more commonly by incorporating a plastic additive which changes color under the effect of laser irradiation. Such laser marking additives may be metal oxides, mixed-metal oxides, or metal compounds in compositions of metal oxides, mixed-metal oxides, or metal compounds as to cause thermal oxidative damage to the plastics or resin in which they are incorporated. Various materials are known in the art that are laser reactive (e.g., capable of changing color when contacted by a laser beam). As described in U.S. Pat. No. 4,861,620, Azuma et al, U.S. Pat. No. 4,753,863, Spanjer, and U.S. Pat. No. 4,707,722, Folk et al, the part or component may be partially comprised of the laser markable material or have a coating of the material on the surface of the part or component to be marked.
However, in most cases, the amount of color contrast which is achieved by known laser marking methods is not as high as desired. Accordingly, there is an ongoing need for additives which can cause significant color changes to occur in the polymeric materials in which they are incorporated. Moreover, it is highly desirable that these additives not deleteriously affect the beneficial physical properties of the polymers. Application of an inert shell consisting of one of the oxides described can mitigate the effect of thermal oxidative degradation and thus increase the functionality of laser marking additives in plastics sensitive to thermal oxidative damage.
Examples of additive materials include lubricants, process aids, cross-linking agents, release agents, colorants, flame retardants, anti-microbial agents, accelerators, inhibitors, enhancers, compatibilizers, stabilizers, blowing agents, foaming agents, conductive or anti-static, dielectrics, and, in a specific application, laser-direct structuring (LDS) additives. Additives for LDS are defined as metal-containing materials that, after structuring, yield a seeded surface for metal deposition. Additives for LDS materials are usually spinel-based metal oxides, such as copper chromium oxide; metal salts, such as copper hydroxide phosphate; organic metal complexes, and the like. Additionally, many other additives are known in the art. A nearly exhaustive overview of the art is published in the Plastic Additive Handbook, 2009, 6th edition, by Hanser Publications.
Some of the problems associated with plastics are their heat sensitivity, their poor wear and mechanical properties, and easy decomposition due to chemical and radiation-based interactions. These interactions can negatively impact the functionality of the finished plastic articles. The presence of excessive thermal oxidative deterioration leads to loss of mechanical strength, embrittlement, discoloration, and premature failure. Frequently, the source of degradation is long-term environmental exposure, but it can also occur in the processing of plastics particularly during processing stages where heat is used to increase flowability prior to producing the finished article. Undesired chemical reactions propagated by heat, include depolymerization or oxidation of the polymer components through interactions with itself, fillers, additives, or environmental and atmospheric chemical components.
Thermal oxidative degradation of plastic is well studied for the recovery and disposal of waste plastic. The precise reaction mechanisms that lead to the depolymerization and oxidation of the plastic components have not been thoroughly elucidated but contributing factors include the presence of acids, exposure to electromagnetic radiation such as UV- and IR-radiation, ozone, and additional internal and external influences. Particularly, the presence of certain metal-containing compositions has been shown to greatly accelerate (“catalyze”) the decomposition of plastic materials. The acceleration is observed as a decrease in the onset temperature of the reaction as measured by a mass loss and release of gaseous by-product driven by exposure of the sample to heat.
Catalysis resulting in thermal oxidative degradation is partially dependent on the composition of the plastic, the types of additives, the degree of thermal exposure, catalytic activity, concentration in plastic, specific surface area, and presence of anti-oxidants. Metals, metal salts, metal-containing compounds, metal oxides, and mixed-metal oxides have been noted to promote the thermal oxidative degradation of polymers, monomeric components of polymers, and finished thermoplastics articles. Notable examples, of metal-containing materials that can catalyze and promote the rate of degradation include oxides, salts, organic complexes, and compounds containing solely or mixtures of V, Zr, Bi, Cu, Co, Ag, Mo, Zn, Nd, Pr, La, Mg, Al, Ru, Ti, Cr, Ce, Mn, Ni, Pd, Pt, Sn, Fe, Sb, and Ca. For specific examples of compositions, see Gupta et. al, Ind. Eng. Chem. Res. (37), 1998, 2702-2712 and Terakado et. al. J. Anal. App. Pyrolysis (91), 2011, 303-309. Other examples are likely known to those skilled in the art of waste plastic reformation, cracking, recycling, recovery, or disposal.
Thermoset plastics and many coating compositions use identical or related resins and share identical reactive chemical functional groups. Thermal oxidative degeneration of these materials can also be promoted in the same process conditions of heating in combination with one of the above metal-containing oxides, salts, organic complexes, and compounds noted above. Additionally, the effect can be particularly pronounced in uncured resins with active epoxy or acrylate groups.
In some instances it is still desirable to include into thermoplastic, thermoset or coating compositions materials that have been demonstrated to negatively impact the plastic or coating through thermal oxidative degradation. Such is the case when the additive materials contain potentially catalyzing metals but contain additional beneficial properties. Examples of such benefits are in coloration of thermoplastics, particularly the incorporation of complex colored inorganic pigments. Common examples include the degradation of polyvinyl chlorides by iron-bearing pigments, copper compounds, and minerals, and for addition of laser marking or LDS additive materials necessary for laser marking and laser direct structuring and where the benefits of other metal-containing fillers and additives are desirable. In these noted instances it is desirable to have a method to incorporate materials into plastic formulations while protecting the thermoplastic from thermal oxidative degradation from the chemical nature of the materials.